A molecular diagnostic test based on immunohistochemical detection of a characteristic protein biomarker will be developed to diagnose basal-like breast cancer (BLBC), similar to those already in routine use for detection of estrogen receptor and human epidermal growth factor receptor 2 (HER2) in breast cancer. The assay will be based on the detection of transcription factor FOXC1 protein utilizing a novel monoclonal antibody optimized for use on formalin-fixed paraffin-embedded breast tumor tissue. FOXC1 has recently been demonstrated by our group to be a characteristic, highly specific tissue level biomarker of BLBC. FOXC1 is also a potential therapeutic target by virtue of its demonstrated critical role in coordinating aggressive cancer traits. This proposal is focused on demonstrating and establishing the proof-of-concept of such a molecular diagnostic assay for the accurate detection of BLBC. Successful completion of this Phase I study is essential prior to undertaking large scale clinical validation of this diagnostic test in Phase II. If successfully validated, the molecular diagnostic test based on FOXC1 protein expression would greatly simplify the diagnosis of BLBC in routine clinical practice owing to the low cost and ease of integration of the proposed testing system into currently existing clinical pathology practice.